1. Field of Invention
This invention relates to image processing. More particularly, this invention is directed to systems and methods for enhancing the color appearance of color-palletized images.
2. Description of Related Art
A large amount of image data found on the World Wide Web and other image data sources uses a palletized image format to reduce the size of the stored image data. One common image format on the World Wide Web is the Graphics Interchange Format (GIF) standard, which incorporates an algorithm that reduces a full color; e.g., a 24-bit, color image to an 8-bit palletized color image. When displayed using the colors available in the palette on a display screen, such as a CRT, palletized images have acceptable quality.
However, when the same images are printed on a color printer, such as a full-color xerographic printer or digital copier, an ink jet printer or a dye sublimation printer, the underlying degradation caused by converting the full-color image data to palletized image data becomes visible and often unacceptable. This degradation is caused by at least two different mechanisms: 1) compared to images on computer monitors, printed images are presented in a larger image size having considerably more pixels in the image; and 2) converting the original full-color image data to palletized image data is an inherently lossy process.
The images printed from such palletized image data generally suffer from severe artifacts caused by palletizing the original full-color image data. Thus, when printing palletized image data, a plausible full-color representation of the original image data needs to be created from the palletized image data to create a printout that will be acceptable to the end-user.
There are a great number of image filtering techniques known in the art. One such filtering technique is the "sgr"-filtering technique, disclosed in U.S. Pat. No. 5,379,122 to Eschbach, herein incorporated by reference in its entirety. As disclosed in the 122 patent, "sgr"-filtering smoothes image data with minimal deterioration of the image sharpness in the resulting image. It should be understood that the term "sgr"-filtering, as used herein, refers more generally to the general group of trimmed filters, such as, for example, the trimmed filters described in Nonlinear Digital Filters, I. Pitas and A. N. Venetsanopoulos, Kluwer Academic Publishing, 1990.
According to the systems and methods of this invention, this reasonable xe2x80x9cprintablexe2x80x9d color image is generated by forming a filtered representation of the palletized image data that has an enhanced color appearance.
However, "sgr"-filtering conventionally requires adjusting the filter parameters, particularly the value of the variable "sgr", for each image. Typically, this adjustment is done manually. However, manually adjusting the filter parameters, such as "sgr", is time consuming and prone to error. Moreover, manually adjusting the filter parameters using a trial and error approach is usually done iteratively with repeated manual settings. Alternately, the filter parameters can be fixed to avoid this manual adjustment. However, using such fixed filter parameters results in less than optimal filtered images.
This invention thus provides systems and methods for enhancing the color appearance of a palletized image.
This invention separately provides systems and methods for filtering palletized image data to enhance the color appearance of the palletized image.
This invention separately provides systems and methods for an adaptive "sgr"-filtering process.
This invention separately provides systems and methods for adaptive -"sgr"filtering that obtains one or more "sgr"-filter parameters from the actual image data.
This invention separately provides systems and methods for globally determining one or more "sgr" parameters by globally analyzing the actual image data.
This invention separately provides systems and methods for modifying the one or more "sgr" parameters on a local basis as a function of local image data.
This invention separately provides systems and methods for globally determining one or more "sgr" parameters by analyzing an image palette.
This invention separately provides systems and methods which enhances the color appearance of a palletized image by adaptive "sgr"-filtering using a "sgr" parameter obtained globally from either the actual image data or a corresponding image palette and which locally modifies the "sgr" parameter based on local image data.
According to one exemplary embodiment of the systems and methods according to this invention, global "sgr" parameters, which provide a good estimate of the noise in the palletized image, are determined from the palletized image data. The global "sgr"-parameters are then locally modified, during the filtering process, for each individual pixel, based on the dynamic range of the "sgr"-filter block centered on the individual pixel. In particular, when the filtering is performed on a color separation basis, the data in one color separation layer can be used to modify the local filter settings in another color separation layer. For example, if two or more of the color separation layers for each individual pixel are within the global "sgr"-filter, the third color separation layer for that pixel might be filtered, even if it is not within the global "sgr" parameter. Additionally, if the dynamic range of the block of pixels surrounding each individual pixel is within the "sgr" parameter, such that there is no reliable edge or high frequency detail information within the block, the "sgr" parameter is modified to result in stronger smoothing.
According to another exemplary embodiment of the systems and methods of this invention, the global "sgr" parameters are determined by examining the color palette of the image only, without needing to examine the entire image content.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.